1. Field of the Invention
The present invention relates to an image transfer sheet for electrophotography for forming (recording) a clear image on an image supporting element by using an image formation apparatus of the electrophotographic method, an image recorded medium using the same, and a method for manufacturing the image recorded medium. More specifically, the present invention relates to an image transfer sheet for electrophotography for forming a printed image. The printed image may be used for image recorded media, such as information recording media containing personal information and image information of the non-contact or contact type. Such image recorded media may be facial-photograph-containing cash cards, employee identification cards, student identification cards, individual membership cards, resident identification cards, various types of driver's licenses, various types of qualification certificates, and the like. The printed image may be use also for RFID tags, and for image recorded sheets for personal identification and image display boards used in medical settings, and for indication labels, and the like. The invention also relates to an image recorded medium using the same, and a method for manufacturing the image recorded medium.
2. Description of the Related Art
In recent years, the image formation technology has developed. Currently, means are known which can form images of the same quality in large quantities at low cost by various printing methods, such as intaglio printing, letterpress printing, planographic printing, gravure printing, and screen printing. Such printing methods are also widely used for manufacturing an information recording medium which stores predetermined information and which can communicate with an external device in a contact or non-contact manner, such as an IC card, a magnetic card, an optical card, and a card which is a combination thereof.
However, for example, the above-mentioned screen printing requires a lot of printing plates corresponding to the number of images to be printed. In the case of color printing, the required printing plates are further increased corresponding to the number of colors. Therefore, these printing methods are not suitable for individual pieces of personal identification information (including the facial photograph, the name, the address, the date of birth, the variety of licenses, and the like).
Against the above-mentioned problem, the most dominant image formation means currently used is the image formation method based on the printer or the like which employs a sublimation-type or melting-type heat-transfer method using an ink ribbon, or the like. However, while these can easily print personal identification information, these still have a problem that increase in the printing speed lowers the resolution, and increase in the resolution leads to decrease in the printing speed.
A heat-transfer method has been proposed which uses an intermediate transfer element for printing onto the image recording medium (as disclosed, for example in Japanese Patent Application Laid-Open (JP-A) No. 5-096871, JP-A No. 7-068812, JP-A No. 8-142365, JP-A No. 8-156302, JP-A No. 9-314875, and JP-A No. 11-291646). However, in any case, a thin colored layer transferred from the ink sheet is formed on the surface of the above-mentioned intermediate transfer element. A good image quality cannot be obtained unless the thin colored layer is properly transferred to the image recording medium. In addition, the image quality basically depends on the irregularities on the surface of the image recording medium. Therefore, in order to improve the adhesion to the image recording medium, a rubber-like elastic layer is provided on the intermediate transfer element. The image is transferred by pressure contact, so that the image quality is maintained. The surface layer of the intermediate transfer elements is basically designed to have releasability. However, because the surface layer has to be able to follow the deformation of the above-mentioned rubber-like elastic layer, a hard surface layer cannot be used. Therefore, specifically, a silicone rubber or fluorine rubber is used for the surface layer.
In image formation (printing) by the electrophotographic method, the surface of the image carrier is electrically charged uniformly, and then subjected to light exposure according to image signals, to form an electrostatic latent image based on the difference in potential between the exposed portion and the unexposed portion. Thereafter, electrostatic development is conducted with a color powder (an image forming material) called toner with the opposite polarity to (or with the same polarity as) that of the charge of the image carrier, thereby forming a visible image (a toner image) on the surface of the image carrier. In the case of a color image, the color image is created by repetition of this process a plurality of times or by processes conducted by a plurality of image formation units disposed in parallel to form a color visible image, and the color visible image is transferred to an image recording medium and fixed (immobilized, in other words, melting of the color powder mainly by heat followed by solidification thereof by cooling).
As described above, by the electrophotographic method, the electrostatic latent image on the surface of the image carrier is electrically formed based on the image signals. Therefore, it is possible to form the same image any number of times, and it is also possible to easily form different images. In addition, almost the entire toner image on the surface of the image carrier can be transferred to the image forming material transfer element or to the surface of the image recording medium. Moreover, a slight amount of toner image remaining on the surface of the image carrier can also be easily removed by means of a resin blade, a brush, or the like; accordingly, it is possible to easily produce prints for high-mix, low-volume production.
In addition, the above-mentioned toner is generally formed by fusing and mixing a thermally fusible resin and a pigment, as well as optional additives such as a charge control agent, and then pulverizing and atomizing the kneaded substance. Further, the electrostatic latent image in the electrophotographic method has a considerably higher resolution compared with the above-mentioned atomized toner, and a sufficient resolution on a par with the resolutions realized by the screen printing and the heat-transfer method using ink ribbon can be expected.
Also, a color image can be obtained by using color toners of four primary colors of cyan, magenta, yellow, and black and then mixing the toner images of the respective colors. Theoretically, the same colors as realized in printing can be reproduced. In addition, in the above-mentioned color toner, the toner resin and the pigment can be relatively freely compounded, thereby enabling easy increase in light shielding property of the image.
In addition, there have been almost no studies on the heat resistance and light resistance of information recording media intended to be used outdoors. Particularly when a driver's license or the like is left in a car and exposed to direct sunlight, fading occurs if the image is a heat-transferred image using a dye as a coloring material. However, when a color image is formed by the electrophotographic method, pigments corresponding to the respective colors of cyan, magenta, yellow, and black used in the color toner have excellent light resistance. Therefore, the light resistance of the images formed by the electrophotographic method is considered to be sufficiently high. Likewise, if a heat-resistant toner is selected, the heat resistance of the image formed on an information recording medium is considered to be high enough to allow the information recording medium to be used outdoors.
On the other hand, the most widely used substrates (cores) used for various types of cards are currently vinyl chloride sheets. This is because vinyl chloride sheets are excellent in printing characteristics in conventional printing machines, because they are also excellent in suitability for embossing (process to raise or lower the character portions), and particularly because they are inexpensive. However, the above-mentioned vinyl chloride sheets have a problem that dioxins are generated when cards are incinerated by using a heating furnace or the like after being disposed of for expiration or the like. From the viewpoint of environmental compatibility, various types of sheet films are currently used more widely as alternatives to vinyl chloride cards.
If embossing is not carried out in the manufacturing of cards, conventional films such as biaxially stretched PET (polyethylene terephthalate) films can be used. However, in order to retain the functions of the conventional cards, embossing is often indispensable. Films currently used for embossing include: ABS resin films and polyolefin resin films, which soften at relatively low temperatures; a modified PET resin film called PETG, which is obtained by copolymerizing at least ethylene glycol, terephthalic acid, and 1,4-cyclohexane dimethanol; and films obtained by integrally forming a modified PET resin film and (a PET film, an amorphous PET resin film, or a polycarbonate resin film).
A method for producing prints has been proposed which uses the above-described electrophotographic apparatus and further uses a transfer sheet (as disclosed in U.S. Pat. No. 3,359,962 and U.S. Pat. No. 3,359,963). However, in the above-mentioned sheet, the surface resistivity of the thermal adhesion layer, which works an image receiving layer, is particularly high. Therefore, transfer of the image forming material to the surface of the sheet may be defective, whereby fine characters are not reproduced. In T-shirt printing, since a large design and a hue are reproduced, use of such a sheet is not problematic. However, it is impossible to reproduce on the sheet delicate information, such as a facial photograph and a two-point character to be recorded on an ID card. In addition, the charge caused by a high resistance is problematic since the charge easily attracts fiber waste, dust, dirt, and the like, which seriously affect the card quality. In addition, when the supporting element is a plastic film, the coefficient of friction between sheets is too high, whereby the sheets tightly adhere to one another and the transferability of the sheets is deteriorated.
On the other hand, a method has been proposed, for example in JP-A No. 2001-92255. In the method, in addition to various types of personal information, an invisible bar-code is printed on a vinyl chloride sheet having a thickness of 250 μm or on a polyester sheet having a thickness of 280 μm by the electrophotographic method; then an over-film is superposed on each printed surface, and lamination is carried out by a hot-press machine.
However, in the above-mentioned sheet, the coefficient of friction between sheets is too high. Therefore, the sheets tightly adhere to one another, thereby deteriorating the transferability of the sheets. This poor transferability may stop the electrophotographic apparatus. Further, an insulator (a sheet) having a thickness of 250 μm or larger as mentioned above may cause an increased image defect because of insufficient transfer of the image forming material (toner) thereto. In addition, when the resin film which softens at a relatively low temperature is used for image formation by an electrophotographic apparatus, the following problem occurs: the tackiness develops during the fixing process because the fixing temperature is higher than the softening temperature of the film; and the film winds around the fixing apparatus, resulting in jamming. Further, the image forming material may offset to the fixing apparatus. Further, when fixing is continuously conducted on the sheet having a thickness of 250 μm or larger, the edge (corner) of the sheet may unnecessarily damage the fixing apparatus, causing necessity for frequently replacement of members.
As another example, a method has been proposed in which a mirror image of personal identification information is printed on an optically transparent sheet (as disclosed, for example in JP-A No. 11-334265). However, with respect to the optically transparent laminate sheet, the JP-A No. 11-334265 only teaches that at least a part the laminate sheet is preferably a biaxially stretched polyester film, ABS, or a film/biaxially stretched polyester film consisting of polyester, but may also be vinyl chloride.
Therefore, in this structure, since the film is simply an insulator, transfer of the image forming material to the surface of the film or the like may be defective, and a resolution cannot be as high as that realized by the heat-transfer method or the like. In addition, in this apparatus focusing on improved productivity, the laminate sheet to be used is in the form of a roll. Therefore, there is a problem that a lot of loss and waste inevitably occur in order to meet the need for urgent or high-mix production, such as when conducting different types of print for cards for one to several persons.
Further, when an information recording medium is manufactured by using these laminate sheets, a plurality of sheets are stacked, thereby increasing the total thickness. For example, it may be difficult to produce an information recording medium having a thickness of about 800 μm.
In addition, there have been almost no studies about the automation of the process of forming an image on the laminate sheet, of feeding and stacking the bases comprised of the laminate sheet having the image and a plastic, or of laminating the bases. Accordingly, it is necessary to design each of the above-mentioned processes and the manufacturing apparatus from the viewpoint of productivity improvement (see, for example, JP-A No. 10-86562).
Further, an image formation method has been proposed which uses a recognition-identification medium obtained by superposing an adhesive layer on a supporting element (a substrate), disposing a transparent sheet on this adhesive layer, and forming an image with a colorant between the supporting element and the transparent sheet. In this method, the total thickness of the image recognition medium is the sum of the thicknesses of the substrate, the transparent sheet, and the adhesive layer. When the image recognition medium is an IC card, a magnetic card, or the like, the respective thicknesses must be controlled to meet the specification of the card thickness (760 μm±80 μm). Particularly the substrate for an IC card contains an IC chip, an antenna, or the like, whereby the thickness of the substrate is often close to the limit of the specification for the card thickness. Therefore, this method has problems such as a problem that the card thickness is increased by the thickness of the transparent sheet and the thickness of the adhesion layer to exceed the specified thickness limit.